Add ability to use nerdfont.vim plugin to display devicons

[theunraveler]

Hello! First, thanks for developing this useful plugin.

I am already using the nerdfont.vim plugin to display dev icons in the fern.vim file browser. I thought it was a bit redundant to install vim-devicons also, so I changed vim-buffet to use either plugin.

The change in this PR should be completely transparent to users of this plugin. The g:buffet_use_devicons setting should continue to function exactly how it currently does, and if you're using vim-devicons, this plugin will still continue to pick that up by default.

Oh also note that a consequence of this change is that the vim-devicons plugin no longer needs to be registered before vim-buffet, since the function for generating devicons is lazily-loaded. So I updated that in the README too.

Thanks!

[krisfans]

It doesn't seem to work on my computer.
my vimrc is

if &compatible
  set nocompatible               " Be iMproved
endif

" Disable Vim's native pack feature
set packpath=

set laststatus=2       " 显示 status line
set showtabline=2      " 显示 tabs line

let g:buffet_show_index = 1
let g:buffet_powerline_separators = 1
let g:buffet_use_devicons = 1
set runtimepath+=~/.cache/vim/dein/repos/github.com/lambdalisue/nerdfont.vim
set runtimepath+=~/.cache/vim/dein/repos/github.com/theunraveler/vim-buffet_nerdfont
filetype plugin indent on
syntax enable
set termguicolors


    " Note: Make sure the function is defined before `vim-buffet` is loaded.
    function! g:BuffetSetCustomColors()
        " hi! BuffetTab cterm=NONE ctermbg=117 ctermfg=238 guibg=#8ac6f2 guifg=#444444
        " hi! BuffetCurrentBuffer cterm=NONE ctermbg=119 ctermfg=8 guibg=#95e454 guifg=#000000
        hi! BuffetTab cterm=NONE ctermbg=117 ctermfg=238 guibg=#a89984 guifg=#282828
        hi! BuffetCurrentBuffer cterm=NONE ctermbg=119 ctermfg=8 guibg=#b8bb26 guifg=#282828
    endfunction